The world-wide production of chlorine, about 45 millions of tons per year, is carried out in electrolytic cells of various types, one of the most relevant being the diaphragm electrolysis cell, whereby about 22 millions of tons per year of chlorine are produced.
An electrolysis diaphragm cell is generally made of four main parts, as widely known to the experts of the art: a copper anodic base lined with a protective titanium sheet, an anodic package made of a multiplicity of anodes arranged in parallel rows and secured to the base, an iron cathodic body comprising a plurality of cathodes with a semipermeable diaphragm deposited thereon, fixed to a current distributor and arranged in parallel rows interposed to the anodes according to a so-called “finger-type” geometry, and a cover, usually made of chlorine-resistant plastic material provided with inlets for feeding brine and outlets for discharging the product chlorine.
In consideration of the high number of operating cells (about 25,000 in the world), of the high amount of energy required for their operation (about 60 millions of MWh/year) and of the continuous increase in the cost of electric energy, the diaphragm cell technology has been considerably improved during these years. Among the various technological improvements contributing to reducing the energy consumption, the following should be mentioned:                replacement of the traditional graphite anodes with perforated box-shaped metal anodes (the so-called “box type” anodes) made of titanium coated with electrocatalytic materials based on noble metals and/or oxides thereof.        the replacement of “box type” anodes, of fixed dimensions, with the so-called “expandable anodes”, as described in U.S. Pat. No. 3,674,676, allowing the reduction of the interelectrodic gap        the achievement of a zero-gap cell design by means of the introduction, within the expandable anodes, of suitable devices for pressing the anodes against the diaphragm, as described in U.S. Pat. No. 5,534,122        the introduction of internal electrolyte recirculation devices, as described in U.S. Pat. No. 5,066,378.        the catalytic activation of the cathode, by means of the application of an activated intermediate element on the cathodic surface or by the catalytic activation of the diaphragm itself.        
As can be observed, the above mentioned improvements are all directed to obtain better performances in terms of energy consumption by increasing the electrocatalytic activity, or by optimising the electrode structure, or again by decreasing the interelectrodic gap and increasing the mass transfer (lower bubble effect and higher electrolyte circulation) achieved by small modifications which do not imply a substantial redesigning of the cell structure and thus can be easily applied with reduced costs.
In many cases, however, it would be preferable to lower the energy consumption by increasing the electrodic surface while keeping the same current load, thereby decreasing the current density and consequently the cell voltage. This situation is typically experienced in the operation of existing cells, due to variations in the price of electric energy, or to the incoming availability of electrical components capable of withstanding a greater load. This can be particularly critical in case of lack of available space on the plant site for installing new electrolytic cells in addition to the existing ones. For this reason, in the past, several solution involving the modification of the cell structure, and in particular of the anodic package and of the cathodic body, have been proposed. Although these improvements entail a very significant energy saving, in the same range of the previously mentioned ones or even higher, they are of lower relevance and commercial success as they involve remarkable modifications of the internal cell structure or variations of the external dimensions, implying strong investment costs and long construction and pay-back times. Among these solutions, the following can be mentioned:    a) increase of the ratio between the electrodic surface and the volume of the cell by internal modifications of the latter, namely:            substitution of the whole cathode package with a new one, having the same overall dimensions but a reduced spacing between fingers (finger pitch), so that a higher number of fingers is installed and a proportional increase of the cathodic surface is obtained.        new perforation of the anodic base to adapt the position of the anodes to the new finger pitch of the cathodic package.        insertion of a number of new anodes, identical to the old ones, between the fingers of the cathodic package giving rise to an analogous increase of the anodic surface.        
No external modifications are conversely required. This method may be applied when there is sufficient space for reducing the finger pitch and is generally applicable to cells of the old technology, conceived for operation with graphite anodes and thus having a higher pitch between one finger and the next; commonly, the increase in the surface that can be achieved does not exceed 2-5% of the existing surface. The investment is economically viable when the cathodic package has to be substituted at the end of its lifetime, which usually happens every 6-8 years. The retrofitting times are therefore long.    b) increase of the cathodic package height and substitution or modification of the existing anodes.
This technique implies substantial modifications inside the cell, including the complete substitution of the cathodic body and the substitution or modification of the existing anodes.
Small modifications are required also outside the cell, especially as concerns the hydraulic connections, even though they are not very significant from an economic standpoint; however this method, although offering the advantage of a greater increase of the electrodic surface (5-15%), is much more expensive and has very long retrofitting times: it is therefore economically interesting only when the cathodic package must be in any case relpaced being close to the end of its operating lifetime (every 12-16 years).
As a conclusion, the two latter methods, although easily applicable from a technical standpoint, have the great disadvantage of being very expensive and entailing long retrofitting times, posing problems of pay-back and being economically convenient only in case of a concurrent substitution of the anodic package or cathodic body. It is an object of the present invention to provide a diaphragm electrolytic cell for chlor-alkali production which overcomes the shortcomings of the prior art.
In particular, it is an object of the present invention to provide a diaphragm electrolytic cell having an increased electrodic area.
Under another aspect, it is an object of the present invention to provide a method for obtaining a diaphragm electrolytic cell having an increased electrodic area starting from a conventional cell.
Under another aspect, the invention comprises an electrolytic diaphragm cell including a plurality of anodic packages arranged on a plurality of overlaid planes. Under a further aspect, the invention comprises a method for increasing the electrodic active area of a diaphragm cell without replacing or removing the pre-existing anodic and cathodic packages.
Under a further aspect, the invention comprises a method for increasing the active area of a diaphragm cell wherein the base surface of the cell is maintained constant.